The invention relates to very large space structures having rotary joints and the requirement for a means to transfer electrical power across the rotary joint such as in transferring electrical power produced by a solar cell structure to a microwave antenna and support structure where the solar cell and microwave antenna support structures have a common axis of rotation.
This type of mechanical/electrical rotary joint design is applicable to space stations, large power satellites, construction bases, and systems utilizing nuclear reactors or solar thermodynamic power sources such as a photovoltaic space power satellite. The invention is particularly applicable to systems with power ratings of between 100 and 10 million kilowatts and the advantages increase with the size and power rating of the system.
The known prior art of rotary joints with power transfer assemblies used in space systems has been limited to small sizes able to support structures weighing a few hundred kilograms and to power ratings of a few kilowatts at about 100 volts. Conventional technology applicable to motor gear drives, small rotational shafts and bearings, and slip rings and brushes has satisfied previous applications. Typical electrical rotary joint apparatus for transferring electrical power is shown in U.S. Pat. No. 3,095,252.
Although improved materials and components have been developed and used, the configuration and overall design approach to such structures has been basically conventional. Considering the size and mass of such structures, the configuration and designs heretofore used have been rather complex and inefficient.
The state of the art for very large rotary joints with power transfer consists of enlarged and extended conventional rotary drive assemblies which include mounting huge slip ring and brush assemblies (over 1 kilometer in diameter) around the outer periphery of the rotary joint structure. In general, these have been very large, massive and rather impractical and operationally and economically infeasible.
A large diameter slip ring as required for a large satellite power system would require a large amount of expensive and scarce slip ring cladding such as silver. The cost of high performance materials for this application is too excessive. The huge circumference of the rotary assembly would incur very high distribution losses and heat rejection requirements. In addition, the brush speed and wear would be much higher than necessary. The large diameter concepts further require complex bearings of unprecedented size and alignment requirements, and the high frictional torque caused by such bearings and brushes are conducive to distortion, especially when large thermal variations are superimposed. The high frictional torques and the distortions incurred impose stringent requirements on the control and stability subsystems. Furthermore, the large conventional slip ring assemblies require orbital assembly and checkout.
The large diameter slip ring assemblies preclude the use of multiple slip rings and brush assemblies needed to subdivide the distribution system into smaller, unrelated and controllable buses within the capacity limits of switch gears. Without such provision, fault protection is inadequate, and the reliability and life of the system is seriously impaired.